Radiator cap for Koyo's FC radiator
 

I just ordered a Koyo AL radiator, Koyo part # R1144N, which fits the series 5 FC's, but I forgot all about the radiator cap... Does anyone know for sure if the stock cap will fit this radiator?

If not, Koyo makes a cap for this radiator, but it's a higher pressure cap than stock - 1.3 vs. 0.9 bar, IIRC. I realize that a higher pressure cap is good from the standpoint of raising the coolant's boilover temp, but was a little concerned the additional pressure might not be good for seals in a rotary or could make leaks more likely -- is this a valid concern?

If everything is healthy the increase in system pressure won't matter. If something is marginal or already failing it may cause it to fail. Personally as long as I had new hoses upper, lower, and heater core (i think those are all the coolant hoses on the car) I'd have no qualms as long as I thought the engine was healthy. If the engine is suspect due a pressure test of the system at 30 PSI and check a steady pressure over a few hours. If you don't hold pressure then you might have an issue that the 1.3bar cap might make worse.

I've got the Koyo N-Flow in my car and I'm using a stock radiator cap on it. (Actually, I'm pretty sure it's an OEM equivalent from an auto parts store, but it wasn't a Koyo specific cap.)

Good to hear -- I'll just use my working Mazda stock cap then. While shopping around for the Koyo, I was getting conflicting information from the various dealers I talked to regarding the necessity of buying the Koyo cap - some said it's OEM, others said you "need" the Koyo cap. Yeah, they "need" you to buy an extra and unnecessary part to fatten their bottom line. :cuss:

I have the same radiator with the cap from my stock radiator. Although I do not have a gauge to tell you the actual difference in temperatures, I can tell you that I had no problems running in local autocross events during the summer.